This invention relates to tuned switching power amplifiers and in particular to a Class E, fixed frequency resonant converter with phase-shift control.
In the prior art a high-efficiency, tuned, single-ended, switching mode amplifier employing an active device switch driver at a frequency determined by an A.C. input signal wherein the active device switch controls the application of direct current power to a load through a resonant load network is disclosed in U.S. Pat. No. 3,919,656, issued to Nathan O. Sokal et al. on Nov. 11, 1975. Sokal et al. describes a tuned power amplifier which avoids by design the simultaneous imposition of substantial voltage and substantial current on the switch, even during switching intervals of substantial duration, through the use of a load network synthesized to yield an optimal transient response to the cyclic operation of the switch; this results in maximizing power efficiency even if the active device switching times are substantial fractions of the AC cycle. Sokal et al describes the load network operation for achieving a high-efficiency tuned switching power amplifier as Class E operation. The optimum operation satisfies the following criteria when the active device switch is a transistor: a) the rise of the voltage across the transistor at turn-off should be delayed until after the transistor is off; b) the collector voltage should be brought back to zero at the transistor turn-on; and c) the slope of the collector voltage should be zero at the time of turn-on. However, this basic tuned power amplifier remains in an optimum mode of operation only within a limited load and input line range. It is generally considered unsuitable for DC-to-DC converter applications since it requires a relatively critical load impedance to keep conduction losses low. Also, the basic Class E converter operates only with frequency modulation (FM) control. In addition, two stages connected in a push-pull configuration cannot operate with fixed frequency phase-shift control while maintaining the optimum mode of operation.
The application of two Class E amplifiers in a push-pull configuration is disclosed in an article entitled "Idealized Operation of the Class E Tuned Power Amplifier" by Frederick H. Raab, IEEE Transactions on Circuits and Systems, Vol. CAS-24, No. 12, December 1977, pp 725-735. Raab shows an arrangement of two basis Class E circuits to form a push-pull Class E amplifier in order to obtain a larger power output. The two circuits are driven with opposite phases via a transformer. Each circuit operates as if it were a single transistor Class E amplifier. The voltage appearing on the secondary winding of an output transformer contains both a positive and negative "Class E" shape. Consequently the output voltage amplitude has twice the amplitude of the signal at the collector of each single transistor Class E amplifier. However, this push-pull Class E tuned power amplifier has a limited load range as does the Sokal et al. basic Class E converter and it cannot regulate the output power using fixed frequency control.
Class E converters operating with frequency modulation (FM) control suffer from a variety of disadvantages. A wide range of switching frequency is often required to maintain output regulation. Wideband noise generated by the converter complicates EMI filtering as well as system design. At light loads, the operating frequency is reduced. This in turn reduces the closed-loop bandwidth of a converter and slows down the transient response. An FM controlled circuit is subject to entrainment which occurs when the FM controller locks on the frequency of a pulsatory load, resulting in an increase in output ripple. Limited data describing this phenomenon makes it particularly risky to supply a dynamic load with negative impedance from an FM regulated converter. Power supplies with FM control can be synchronized only when they share a common load. Unsynchronized converters feeding sections of a densely packaged system can generate broadband beat frequencies. If these frequencies are within the passband of a power supply, a significant increase in the output ripple voltage may result. The control characteristic of the FM control is non-linear in the continuous conduction mode, i.e., the small-signal gain drastically changes when the load current is changed. To avoid these disadvantages of FM control, fixed frequency control has been proposed.
In order to overcome the disadvantages of a Class E resonant converter with FM control, fixed frequency control is described in an article entitled, "Steady State Analysis of Class E Resonant DC-DC Converter Regulated Under Fixed Switching Frequency" by Koosuke Harada and Wen-Jian Gu, Power Electronics Specialists Conference, April 1988, pp. 3-8. Here, Harada et al. describe a Class E resonant DC-DC converter which is regulated by an auxiliary switch and the switching frequency of the converter is fixed. Auxiliary switches are used to modulate resonant frequency in order to regulate resonant converters. However, this approach does not provide output regulation at light loads.
In an article entitled "Class-E Combined-Converter by Phase-Shift Control" by Chuan-Qiang Hu et al., Power Electronics Specialists Conference, PESC '89, June 1989, pp. 229-234, a Class E combined-converter is described which can be easily regulated for both wide load and wide line voltage variations while the switching frequency and the tank resonant frequency are both fixed. Such converter is a parallel combination of two conventional Class E converters. Both units operate at the same switching frequency with an adjustable phase-shift angle .alpha. from 0.degree. to 180.degree. between them allowing control of the output power. The feedback control is claimed to be simpler than that described in the Harada article. However, this Class E combined converter has poor regulation at light loads, but the light load regulation of this circuit is several times better than the regulation of the Harada et al. circuit. In addition, as is common to all converters with vector summing control, one of the section inverters must be designed somewhat oversized to account for a poor apparent load power factor.